Devices that store data are key components of any computer system. Computer systems have many different types of devices where data can be stored. One common device for storing massive amounts of computer data is a disc drive. The basic parts of a disc drive are a disc assembly having at least one disc that is rotated, an actuator that moves a transducer to various locations over the rotating disc, circuitry that is used to write and/or read data to and from the disc via the transducer, and a bus interface to connect the disc drive into a data-handling system, such as a host computer. The disc drive also includes circuitry for encoding data so that it can be successfully written to, and retrieved from, the disc surface. A microprocessor controls most of the operations of the disc drive, including operations that pass the data back to a requesting computer and receive data from a requesting computer for storing to the disc.
A typical magnetic disc drive includes a transducer head for writing data onto circular or spiral tracks in a magnetic layer the disc surfaces and for reading the data from the magnetic layer. In some drives, the transducer includes an electrically driven coil (or “write head”) that provides a magnetic field for writing data, and a magneto-resistive (MR) element (or “read head”) that detects changes in the magnetic field along the tracks for reading data. Some MR elements include giant magneto-resistive (GMR) technology.
The transducer is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (“ABS”) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air-bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring that produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc-drive systems read and/or write information stored on tracks on storage discs. Transducers, in the form of read and/or write heads attached to the sliders, located on both sides of the storage disc, read and/or write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. An example of a disc drive that only reads data (and thus only includes a read head) is a compact disc read-only memory (CDROM) drive. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of a disc drive. Servo positioning information written on the disc(s) is used to accurately locate the transducer.
The transducer is also said to be moved to a “target track.” Once the storage disc spins and the write head is accurately positioned above a target track, the write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track in an operation called a “seek.” A seek is movement of an actuator assembly from a first track to a second target track.
One or more buffer memories are typically provided within the disc drive. These buffer memories are used to receive data from the disc transducer(s) at one rate and period of time, to send data to other buffers within the disc drive and external to the disc drive at another rate and/or period of time, and as temporary storage for operations on the data itself. Such operations on the data itself can include error correction operations such as correcting errors by using redundant information in the data, or adding such redundant information so that errors occurring later may be corrected. By receiving data at one rate and transmitting data at another rate, the buffer memories match the respective data speeds. For example, data is received from the disc at a rate determined by the rotational speed of the disc or the data density on a particular track, either or both of which can vary from the inner diameter of the disc to the outer diameter. Data is sent to the host computer or data-handling system at a speed determined by the host interface.
High-speed busses can have several data channels. A data channel will typically “burst” data to the bus for a short period of time, and then release the bus for use by other channels. Bus hogging can occur if one channel on the bus performs a data burst that takes a relatively long time, thus depriving other channels of access to the bus to perform their data transfers, and increasing their data latency.
There is also a need for an inexpensive method and apparatus that limits bus hogging and provides fair access to critical bus resources.